1. Field of the Invention
This invention relates to the production of sodium carbonate (soda ash) from natural ores such as trona, and more particularly to an improved process for producing two grades of soda ash product, one more pure than the other.
2. Description of the Prior Art
While a considerable quantity of soda ash is produced in accordance with the well-known Solvay process, increasing amounts have of late been produced from natural alkali metal carbonate-containing mineral deposits, primarily located in the Green River Basin of Colorado, Utah and Wyoming. Examples of such minerals are trona, dawsonite and nahcolite. Trona, as mined in the area of Green River, Wyoming contains about 90 to 95 percent sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3. 5 H.sub.2 O). The remaining s to 10 percent consists mainly of insoluble components (principally shale) plus sodium chloride (NaC1), sodium sulphate (Na.sub.2 SO.sub.4), and small quantities of organic matter.
In a common method of obtaining marketable soda ash from these minerals such as trona ore, the crude trona is first heated to approximately 150 to 200.degree. C. to decompose the bicarbonate portion of the sesquicarbonate into sodium carbonate carbon dioxide. The resulting crude soda ash is mixed with enough water in a dissolver to produce a substantially saturated solution having about 30 percent Na.sub.2 CO.sub.3 by weight. The solution is separated from the insoluble impurities, treated with activated carbon to remove a majority of the organic matter, and then usually filtered to yield a purified clear pregnant liquor, but which still contains some organic matter plus dissolved NaCl and Na.sub.2 SO.sub.4.
The pregnant liquor is then fed into multiple effect evaporative crystallizers (usually three-stage) in which water is evaporated and crystals of sodium carbonate monohydrate (Na.sub.2 CO.sub.3.H.sub.2 O) are formed. The crystals are separated from the mother liquor and dried to form sodium carbonate. The sodium carbonate solid is screened and the insize fraction is then stored for shipment as final product. Because the mother liquor may contain as much as 30 percent of the sodium carbonate in the original pregnant liquor feed, it is normally recycled to the crystallizers for further treatment.
The basic process outlined above, also known as the monohydrate process, is disclosed in U.S. Pat. No. 2,962,348, issued to L. Seglin et al on Nov. 29, 1960. U.S. Pat. No. 3,933,977, issued to J.M. Ilardi et al on Jan. 20, 1976, provides additional detailed description of equipment and procedures for preparing a pregnant liquor carbonate process solution, and the disclosures of both these patents are expressly incorporated here by reference.
Although recycling the mother liquor greatly improves the yield of sodium carbonate crystals, both the concentration of any residual organic matter which passes through the carbon treaters and soluble impurities, such as NaCl and Na.sub.2 SO.sub.4, tend to build up in the crystallizers. If the organic matter buildup becomes too high, it can cause foaming in the crystallizers and the sodium carbonate crystals will be discolored and possess undesirably low bulk density. Excessive concentrations of NaCl and Na.sub.2 SO.sub.4 will result in the formation of complex salts which may crystallize out along with the sodium carbonate to cause off-specification product. Although the foaming effect can be offset to some extent by adding defoaming compounds, the adverse effects on the crystalline product can be prevented only by keeping the concentration of these impurities at a relatively low level.
One proposed solution to this problem is to recycle only a portion of the mother liquor and purge the rest. The previously mentioned Seglin et al U.S. Pat. No. 2,962,348, for example, teaches puring enough mother liquor from the recycling line to operate at a concentration of about 5% combined NaC1 and Na.sub.2 SO.sub.4.